Digital Signal Processing (DSP) is being implemented extensively, due to the advances in the VLSI technology. The fabrication of high speed and high complexity DSP circuitry can now be accomplished in a cost effective manner by using subricron geometry processes.
One large area of DSP is digital filters. Digital filters have found application in systems which used to be implemented predominately in the analog domain. An example of application where digital filters are used is the Sigma Delta Analog to Digital Converter (.SIGMA..DELTA. ADC). The .SIGMA..DELTA. ADC is an oversampling converter where the input is sampled at a much higher rate than the Nyquist frequency. The ADC typically consists of two parts: the Sigma-Delta modulator and the digital filter. The modulator converts the analog input into a digital bit stream. The bit stream output of the modulator could be 1 bit or a few bits. The modulator also shapes the quantization noise such that the energy of the low frequency noise is very low and the energy of the high frequency noise is increased due to filtering the modulator loop. The digital filter used in this architecture serves two purposes. First, it suppresses the high frequency noise produced by the modulator. Hence, it is generally a low pass filter. Second, the digital filter converts the single bit or a few bit wide digital output of the modulator into a N bit digital number. The number N is determined by the desired resolution of the system. The typical value of N is from 8 to 20 bits.
The conversion time of the .SIGMA..DELTA. ADC system is limited by the digital filter due to its low bandwidth for high resolution. In order to achieve a desired resolution for a given system, the digital filter needs to accumulate a pre-determined number of samples. The resolution of the system dictates the number of data samples. Thus, for higher resolution systems, a relatively long time is required due to long filtering time for the ADC.
For example, in many battery operated systems, the device is periodically powered on momentarily to acquire the data and the power is conserved. However, the system has to be powered long enough to allow the output to reach the final value within a desired error band. Thus, it is imperative to have fast settling .SIGMA..DELTA. ADC for low power requirement application.
One such system is a gas flow and leak detection device that consumes 200 uA and has a conversion rate of 5 ms. The application requires data to be collected every second. Thus, once every second, the device needs to be powered on for 5 ms. The average current is thus (200 uA * 5 ms)/1s=1 uA. If a faster filter is implemented on the device that shortens the conversion time to 4 ms while the current consumption remain the same, the average current for the device can be lowered to 0.8 uA, a 20% saving. A battery with capacity of 100 mA*hour can power the device for 14.3 years instead of 11.4 years, a 25% improvement in battery life. Since only authorized personnel are allowed to replace the battery, a longer battery life is very significant cost in the application. This can have a tremendous cost savings as this gas flow and leak detector is placed on four million households. There are many other applications where frequent battery replacement is costly or troublesome for example, tire pressure measurement device that is mounted inside the tire; pacemaker; air plane "black box" recorder, etc. The improvement of battery life time due to faster filter would have significant impact in such applications.
Accordingly, what is needed is a method and system for providing a digital filter that rapidly reaches the desired value in spite of a long time constant of the filter. Thus, the new system will be low noise (longer time constant) but a very fast response time as the power is turned on. Thus, the number of samples required will be dramatically reduced to a smaller number. The present invention addresses the design of such a filter scheme that will reduce the settling time and keep the low bandwidth. The same idea can be extended to band pass filter or any other types of noise shaping filters. Furthermore, this novel idea can be implemented with analog and mixed signal circuitry.